Volume Calibration

Volume calibration determines the volume a volumetric instrument delivers or contains. The method is mostly gravimetric: the water dispensed by the instrument is weighed and converted to volume using the density of water at that temperature. In other words, volume is measured via mass, not directly.

The aim is for the volume transfer that analytical results rest on to be accurate. A pipette's systematic error carries through to every solution prepared with it, and hence to every analytical result.

• Automatic (micro) pipettes — single- and multi-channel
• Burettes and dispensers
• Volumetric flasks and graduated cylinders
• Volumetric pipettes
• Automatic dilution and dosing systems
• Syringe pumps

• Systematic error (accuracy): the deviation of the mean volume from the nominal value
• Random error (repeatability): the spread of repeated deliveries
• Behaviour at specific percentages of nominal volume (typically low, mid, high)
• The difference between channels in multi-channel pipettes
• The temperature correction and water density

The calibration interval is not fixed by any standard and there is no general rule such as "once a year". The interval is set by weighing together the manufacturer's recommendation, how often and under what conditions the instrument is used, how critical the measurement is, the drift shown by the instrument's previous calibration results, and any legal requirement. ILAC-G24 / OIML D10 describe the methods for setting it. The sound approach is to start with a reasonable interval for the first period and then lengthen or shorten it in the light of subsequent results: an instrument that proves stable can have its interval extended, one that drifts has it shortened.

• ISO/IEC 17025
• ISO 8655 series (piston-operated volumetric instruments)
• ISO 4787 (volumetric glassware)
• ISO 3696 (water used in calibration)
• JCGM 100 (GUM — uncertainty)

• ISO/IEC 17025
• ISO 8655 series (piston-operated volumetric instruments)
• ISO 4787 (volumetric glassware)
• ISO 3696 (water used in calibration)
• JCGM 100 (GUM — uncertainty)

1. 1Receipt of the Instrument and Initial InspectionThe cleanliness and physical condition of the instrument are checked. On volumetric glassware, cracks and wear are examined; on flow meters, residues.
2. 2Conditioning of Environment and FluidThe instrument and the fluid to be used are brought to equilibrium under laboratory conditions. Temperature directly affects the density of the fluid and therefore the result.
3. 3Preparation of the Reference SystemA reference flow meter, gravimetric rig or acoustic calibrator with documented traceability is put into service.
4. 4Selection of the Measurement PointsPoints representing the instrument's operating range are selected; in flow calibration the low-flow region is addressed separately because error grows there.
5. 5Taking the MeasurementsAt each point, repeated measurements are taken once a steady flow or steady signal has been established.
6. 6Application of CorrectionsTemperature, pressure and density corrections are applied; in the gravimetric method the air buoyancy correction is taken into account as well.
7. 7Uncertainty BudgetThe reference uncertainty, repeatability, temperature stability and reading resolution are combined.
8. 8Issuing the CertificateThe results and uncertainty values are entered on the certificate; a statement of conformity with the decision rule is added if requested.

Every calibration result is given together with a measurement uncertainty; a result reported without its uncertainty is incomplete. The uncertainty is budgeted following the GUM (JCGM 100) approach: the uncertainty of the reference, repeatability, resolution, environmental effects and the stability of the instrument are combined. The certificate states the expanded uncertainty with the coverage factor (usually k = 2) and the approximate level of confidence. The smallest uncertainty a laboratory can report is limited by its calibration and measurement capability (CMC) in the accredited scope; no better uncertainty may be declared.

• The balance's uncertainty and resolution
• The measurement uncertainty of water temperature and the density conversion
• Evaporation loss (especially at small volumes)
• The air-buoyancy correction
• Ambient temperature, humidity and pressure
• Operator technique and repeatability

• Laboratory details and certificate number
• The customer and identification of the instrument calibrated: make, model, serial number
• Date of calibration and date of issue of the certificate
• The calibration method applied or reference to the procedure
• Reference standards used and traceability information
• Ambient conditions during calibration (temperature, humidity and, where relevant, pressure)
• Measurement points, reference values and instrument readings
• The deviations calculated
• Expanded measurement uncertainty and coverage factor
• The personnel who performed and who approved the calibration
• The calibrated volume points (as a percentage of nominal volume)
• The water temperature and the density value used
• Ambient temperature, humidity and pressure
• Whether the instrument is 'Ex' (to deliver) or 'In' (to contain)
• Per-channel results for multi-channel pipettes

A calibration certificate is NOT a certificate of conformity. Calibration measures and reports the deviation of the instrument; it does not correct that deviation and does not say that the instrument is "correct". To decide whether an instrument is fit for use, a tolerance (acceptance criterion) must be defined — and it is the user who sets that tolerance, not the laboratory. If a statement of conformity is requested, ISO/IEC 17025 clause 7.8.6 requires a DECISION RULE to be agreed in advance, defining how measurement uncertainty is taken into account in the decision; a conformity statement made without regard to uncertainty is misleading. The certificate also shows the condition AT THE TIME of calibration: later use, transport or shock can change the instrument's condition, and the certificate does not cover that. Where a next calibration date appears on the certificate, it is not the laboratory's decision but a reminder entered according to the interval stated by the customer.

Calibration is carried out under ISO/IEC 17025, which requires the laboratory's technical competence, impartiality and metrological traceability. Accreditation is granted per METHOD and per MEASUREMENT RANGE: a quantity or range that does not appear in the laboratory's scope annex is not accredited, and no certificate bearing the accreditation mark may be issued for that measurement. The scope also states the smallest uncertainty the laboratory can offer (CMC); no better uncertainty may be declared. EUROLAB holds internationally recognised accreditation under ISO/IEC 17025, granted by accreditation bodies that are signatories to the ILAC mutual recognition arrangement; whether your instrument can be calibrated within accredited scope is confirmed in writing at the quotation stage.

• Flow Calibration
• Acoustic Calibration
• Photometry Calibration